I received an email this afternoon from Altova, announcing the release of XMLSpy 2004. I evaluated the previous version, which seemed to make dealing with XML on win32 much easier. It was also quite useful for evaluating XSLT.
It looks like XMLSpy 2004 has support for editing WSDL, which could be quite useful if applied to platforms with limited or nonexistant WSDL support.
Along those same lines, xmlhack (via Mark) mentions nXML. nXML is an open source mode for Emacs that allows “context-sensitive validated editing against Relax NG schemas.”